Certain isolates of the plant-pathogenic fungus Stemphylium botryosum produce a phytotoxin, stemphyloxin I. This toxin (C21H 34O6) was crystallized and its structure was determined by x-ray crystallography to be a β-ketoaldehyde trans-Decalin. This compound is a highly unusual natural product. Iron (Fe3+) controls production of toxin by this fungus. Furthermore, iron reacts with the toxin to yield a colored product which aids in its detection on chromatograms and in its quantitative estimation by colorimetry.